Heat curing epoxy resin compositions containing a reaction product of a polyurethane polymer terminated by isocyanate groups with a monohydroxy epoxide as impact modifier are known and are described, for example, in WO 2004/0055092 A1 and in WO 2005/007720 A1. Such heat curing epoxy resin compositions can be suitable as structural adhesives for applications having very demanding requirements in terms of the mechanical properties.
For reasons of reducing materials consumption, associated cost savings, for reducing weight, and/or for changing properties of a composition or for filling hollow spaces for acoustic insulation or reinforcement, compositions used as adhesives or sealants have been foamed for a long time. For the same reasons, foam heat curing epoxy resin compositions have been used.
Such heat curing epoxy resin compositions can be highly viscous and have hitherto been foamed using chemical or physical blowing agents which develop their action after application and before or during curing of the composition under the action of heat. These foaming processes involve precise application of the foam which is virtually impossible. The increasing volume of the heat curing composition and the direction in which the foam expands when it is produced by chemical or physical blowing agents after application cannot be predicted very accurately. The expanding foam can, for example, push the substrates to be adhesively bonded against one another or swell out from joints to be sealed, so that unsatisfactory end products are formed. Since the expansion of the composition by chemical or physical blowing agents involves an increase in temperature, curing of the heat curing foam can occur simultaneously with foaming. This can result in the heat curing epoxy resin composition being cured before it is completely foamed, which can be undesirable when, for example, the foam is to be used as sealant for joints and the joint is not completely filled due to unsatisfactory foaming. Also, when, for example, adhesively bonding two substrates, corrections such as moving the substrates which have already been joined may no longer be possible after foaming because curing of the epoxy resin composition has commenced during the foaming process. For the same reason, processes such as, for example, “stripping” (i.e., smoothing of sealed joints after foaming) can be impossible or at least more difficult.
In addition, the degree of foaming can be difficult to set and cannot be checked before application.